Wireless local area networks (WLANs) are gaining in popularity, and new wireless applications are being developed. The original WLAN standards, such as “Bluetooth” and IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, envisions data rates up to 54 Mbps over short distances in a 5 GHz band, while 802.11b defines data rates up to 22 Mbps in the 2.4 GHz band. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.
The theoretical capability of new WLAN technologies to offer enormous communication bandwidth to mobile users is severely hampered by the practical limitations of wireless communications. Indoor propagation of radio frequencies is not isotropic, because radio waves are influenced by building layout and furnishings. Therefore, even when wireless access points are carefully positioned throughout a building, some “black holes” generally remain—areas with little or no radio reception. Furthermore, 802.11 wireless links can operate at full speed only under conditions of high signal/noise ratio. Signal strength scales inversely with the distance of the mobile station from its access point, and therefore so does communication speed. A single mobile station with poor reception due to distance or radio propagation problems can slow down WLAN access for all other users in its basic service set (BSS—the group of mobile stations communicating with the same access point).
The natural response to these practical difficulties would be to distribute a greater number of access points within the area to be served. If a receiver receives signals simultaneously from two sources of similar strength on the same frequency channel, however, it is generally unable to decipher either signal. The 802.11 standard provides a mechanism for collision avoidance based on clear channel assessment (CCA), which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel. In practice, this mechanism is of limited utility and can place a heavy burden on different BSSs operating on the same frequency channel.
Therefore, in high data-rate 802.11 WLANs known in the art, access points in mutual proximity must use different frequency channels. Theoretically, the 802.11b and 802.11g standards define 14 frequency channels in the 2.4 GHz band, but because of bandwidth and regulatory limitations, WLANs operating according to these standards in the United States actually have only three different frequency channels from which to choose. (In other countries, such as Spain, France and Japan, only one channel is available.) As a result, in complex, indoor environments, it becomes practically impossible to distribute wireless access points closely enough to give strong signals throughout the environment without substantial overlap in the coverage areas of different access points operating on the same frequency channel.
Access points in a WLAN system are typically interconnected by a wired LAN to communicate with a hub. The LAN serves as a distribution system (DS) for exchanging data between the access points and the hub. This arrangement enables the mobile stations to send and receive data through the access points to and from external networks, such as the Internet, via an access line connected to the hub.
Most commonly, the LAN used as a DS is an Ethernet LAN, operating in accordance with the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method of media access control (MAC) defined in IEEE Standard 802.3 (2000 Edition), which is incorporated herein by reference. The terms “Ethernet,” “CSMA/CD” and “802.3” are used in the art interchangeably to refer to LANs of this type. Ethernet LANs are typically capable of carrying data at high speeds—greater than the aggregate speed of wireless communications between the access points and mobile stations. For example, a 100BASE-T Ethernet LAN is capable of carrying data over twisted pair cabling at 100 Mb/s. Message latency on the LAN is relatively high, however, generally on the order of milliseconds, due in part to the lack of a fragmentation mechanism at the 802.3 MAC layer. Another factor contributing to latency in Ethernet LANs is that the minimum frame size permitted by the standard is 64 bytes (plus 8 more bytes for the frame preamble and start frame delimiter), while the maximum frame size is more than 1500 bytes. Both preamble and minimum frame size are required for backward compatibility with legacy 10 Mb/s Ethernet over half-duplex media.